Common parasite controls immune system

A forced partnership between parasite and host allows Toxoplasma gondii to invade the bloodstream, break into the brain, and prompt behavioral changes from recklessness to neuroticism.

The highly contagious protozoa that infects more than half the world’s population evades the body’s defenses by hacking immune cells, making it the first known parasite to control its host’s immune system. Most of those infected never purge the intruders.

“Toxoplasma is an especially promiscuous parasite,” says Eric Denkers, professor of immunology at Cornell University. “It infects nearly all warm-blooded species, most nucleated cell types, and much of the human population. Although it lives in vital brain and muscle tissues, it usually causes no obvious reaction.